1. Field of the Invention
The present invention relates to an exposure mask which is utilizable for forming a desired pattern by an exposure in manufacture of an electronic component such as a semiconductor device.
2. Description of the Prior Art
In the technical field to execute an exposure by the use of an exposure mask, the processing dimensions tend to be further finer year after year. For example, in the use of an exposure mask for forming a desired pattern in manufacture of a semiconductor device, the patterning based on such exposure is required to be dimensionally more minute in conformity with miniaturization of the device to be manufactured.
In an exemplary case of a semiconductor integrated circuit, the required processing dimensions are gradually miniaturized from year to year and, in the latest technical developments and studies, the dimensional requisite is fundamentally smaller than 0.5 micron. For realizing such extremely fine work, some designs are in progress to attain a higher numerical aperture and a shorter wavelength with regard to an exposer as well as to obtain an improved resist material, and considerable effects have been achieved. In such technical circumstances, some attempts are currently carried out to accomplish extremely fine work beyond the threshold resolution by devising an improved exposure mask (reticle) employed for pattern transfer. Of such technical development, a phase shifting method is now attracting particular attention (as disclosed in Japanese Patent Laid-open No. 58 (1983)-173744; and Marc D. Levenson et al., "Improving Resolution in Photolithography on Electron Devices", Vol. ED-29, No. 12, December 1982, pp. 1828-1836).
The conventional phase shifting method known heretofore will now be described below with reference to FIGS. 2 and 3. Here, an explanation of the above technique will be given with regard to an exemplary case of forming a line-and-space pattern. In an ordinary mask, as shown in FIG. 3 (a), light shielding portions 2 are formed by the use of a light shielding material such as chromium on a transparent substrate 1 of quartz or the like, and an arrangement of repeated line-and-space patterns is formed to produce an exposure mask. The intensity distribution of the light transmitted through such exposure mask is represented by a curve A1 in FIG. 3 (a), wherein the intensity is zero in the light shielding portion 2 while the light is transmitted through the other portions (light transmitting portions 21, 22). Viewing one light transmitting portion 21 as an example, the intensity of the light transmitted therethrough and irradiated to a work member to be exposed is so distributed as represented by a curve A2 in FIG. 3 (a), wherein hill-like maximums are existent at the feet on both sides due to the diffraction of the light and so forth. The light passed through the light transmitting portion 22 is represented by a one-dot chain line. When the light rays obtained through the individual transmitting portions 21, 22 are mutually combined, the light intensity distribution is deprived of its sharpness as indicated by a curve A3 to consequently blur the image due to diffraction of the light, hence a sharp exposure fails to be provided. In contrast therewith, if the phase shifting films 3 are provided on the light shielding portions 21, 22 of the repeated patterns either alternately as shown in FIG. 3 (b) or in a manner illustrated in FIG. 2, any blur of the image resulting from diffraction of the light is eliminated by inversion of the phase to eventually achieve transfer of a sharp image, thereby improving the resolution and the focusing allowance. More specifically, when a phase shifting film 3 is formed on one light transmitting portion 21 as shown in FIG. 3 (b) in such a manner as to cause a phase shift of 180.degree. for example, the light passed through the phase shifting film 3 is inverted as represented by a curve B1. Meanwhile the light obtained through the adjacent light transmitting portion 22 is not passed through the phase shifting film 3, so that none of such phase inversion is induced. Therefore, on the work member to be exposed, the mutually phase-inverted light rays cancel each other in the position B2 at the foot of the light intensity distribution, whereby the distribution of the light irradiated to the work member is rendered ideally sharp as represented by a curve B3 in FIG. 3 (b).
In the example mentioned, the greatest advantage is attainable by causing a 180.degree. inversion of the phase to ensure-the above-described effect. However, for realizing such a satisfactory result, it is necessary for the phase shifting film to have a sufficient thickness ##EQU1## (where n denotes the refractive index of the phase shifting film, and .lambda. denotes the wavelength of the exposure light).
In the process of forming a pattern by an exposure, it is customary that a member used for reduced-size projection is termed a reticle, and a member for life-size projection is termed a mask; or a member corresponding to an original sheet is termed a reticle, and a member obtained by duplicating such a reticle is termed a mask. In the present invention, any of the masks and reticles classified by such various definitions is termed a mask in general.
In the phase shifting exposure mask mentioned above, there is the necessity of forming both a light shielding portion 2 and a phase shifting portion 3, and moreover exact mutual positioning is required in the step of forming them, whereby some complication is unavoidable in manufacture of the exposure mask. Namely, in the conventional technique, there is needed a complicated process of executing a second positioned EB (electron beam) exposure for the mask where a light shielding portion has already been formed by a first EB exposure and etching. For this purpose, a positioning mark must be previously formed at the time of the first EB exposure, thereby inducing intricacy in the mask manufacturing process.